The present invention relates to pneumatic tires, and relates more particularly to pneumatic tires intended for use in heavy, off-the-road equipment.
Throughout this disclosure, the directions and orientations of elements of tires will be described in terms of their orientation relative to the tire itself. Thus, the term "axially," when used to describe the orientation of an element of a tire, should be understood as meaning "in a direction parallel to the axis of revolution of the tire." The term "circumferential" should be understood as meaning "along the circumference of the tire". Also, the relative positions of tire elements will be described as "radially inward" or "radially outward" of one another. Thus, if a first element of a tire is located radially inwardly of a second element, it should be understood that the first element lies closer to the axis of revolution of the tire than the second element.
Pneumatic tires for off-the-road equipment, such as graders, scrapers, front-end loaders and the like must meet severe service requirements. They are exposed to the constant threat of damage from rocks and other sharp objects on the ground. They must support the enormous weight of the equipment, while transmitting substantial "tractive forces." "Tractive forces" are forces such as those which arise in braking and acceleration of the vehicle and which are directed circumferentially of the tire.
For example, one type of known front-end loader has a gross weight of 96,000 pound in the fully loaded condition. Because the weight of the vehicle is sometimes distributed unevenly among the tires, each of the four tires to be used on such a vehicle must be capable of supporting a load of about 46,000 pounds. Each such tire must be capable of transmitting a tractive force of about 29,000 pounds. If the tires do not transmit the tractive forces to the ground, the wheels of the vehicle will simply spin uselessly. Of course, these tires must be capable of operation even under adverse conditions such as mud, loose sand, and snow.
Several basic approaches have been taken to the design of such tires. In the most conventional approach, a bold ribbed tread is molded integrally with the tire. The tread, consisting of molded rubber ribs, engages the ground surface. However, the ribs of the tread may not dig into the ground to adequately transmit the tractive forces. Also, sharp objects can penetrate the tread and rupture the tire carcass.
In another approach, as set forth in U.S. Pat. No. 3,612,624, issued to Stedman, the tire is provided with a plurality of metal plates arranged around its periphery. These plates are joined by link pins to form an endless chain of ground-engaging elements extending around the circumference of the tire, radially outwardly of the tread. The link pins fit into slots defined by axially extensive ribs of the tire tread. The link pins engage the ribs of the tread to provide a positive mechanical drive connection between the tire carcass and the endless chain of plates. This approach has several disadvantages. For one, the link pins and the plates are subject to wear at each joint. This wear is aggravated by the intrusion of dirt into the joints during operation. It is extremely difficult to prevent such intrusion, as off-the-road equipment tires are constantly exposed to dirt. For another, the chain of links and plates is relatively complex, and is difficult to maintain or repair under field conditions.
A third approach is set forth in U.S. Pat. No. 3,773,394, issued to Grawey, and in U.S. Pat. No. 3,871,720, issued to Mosshart et al. In this approach, the tire is also provided with a plurality of plates or tractive elements arranged around its periphery. However, rather than being linked together into a chain, the plates or tractive elements are attached to an endless belt which lies radially inwardly of the plates or tractive elements but radially outwardly of the tire. Retainer elements lie radially inwardly of the belt, and each tractive element is attached to a retainer element. The belt is clamped between each tractive element and the attached retainer element so that the belt links all of the tractive elements and all of the retainer elements together. The tire has a smooth tread without ribs, and it is constructed and arranged so that at least its radially outwardmost or crown region is radially and circumferentially extensible. Thus, under the influence of inflation pressure, the crown region of the tire expands until it bears against the retainer elements. The frictional engagement between the outer surface of the crown region of the tire and the retainer elements links the assembly of the tractive elements, belt, and retainer elements to the tire.
Although this approach avoids the problems of compexity and wear at the joints associated with the chain of plates and link pins of the aforementioned approach, it is subject to other difficulties. Because the assembly of the tractive elements, retainer elements, and belt is merely frictionally engaged with the tire carcass, this assembly can slip circumferentially of the carcass under tractive loads during operation. To overcome this difficulty, the inflation pressure within the tire may be increased so that the crown region of the tire bears against the retainer elements with greater force. For example, a typical tire constructed in accordance with this approach employs an inflation pressure of 130 pounds. However, there are practical limits beyond which the inflation pressure within the tire may not be increased. As will be appreciated, the inflation pressure within the tire tends to pull the belt taut, so that if the inflation pressure within the tire were increased without limit, the stress on the belt would eventually be so great as to cause it to break. In addition, the stress applied to the belt by the inflation of the tire is increased still further by the tractive loads which the belt must transmit. Also, the greater the inflation pressure, the greater the stress imposed on the tire sidewalls, and the greater the chance of a destructive and potentially dangerous tire explosion. Finally, as the inflation pressure within the tire is increased, the tire becomes less flexible and will not insulate the vehicle from shocks generated when the tire encounters a bump in the ground surface, which not only can cause operator discomfort but can also place additional stresses on the frame and other components of the vehicle.